Integrated circuits are commonly used in a wide variety of fields. For example, integrated circuit dynamic random access memories ("DRAMs") are commonly used in computer systems and other devices. Interfacing integrated circuits, such as DRAMs, to other components can be a complex task, particularly when one is attempting to operate the integrated circuit at maximum performance. To assist in the design of electronic systems using integrated circuits, integrated circuit manufacturers publish data books or data sheets that specify the performance parameters of the integrated circuits. For example, a data sheet for a DRAM may specify an access time, i.e., the time required for valid data to be presented to the data bus after an address has been applied to the device, an address hold time, i.e., the time that an address must be present on the address bus after an address strobe signal, and a large number of other timing parameters. Although timing parameters are generally specified as times or ranges of times, e.g., microseconds and nanoseconds, they may also be specified in terms of numbers of clock cycles. For example, the read latency of a memory device may be specified as a number of clock cycles between addressing the memory device and the availability of data from the addressed location on the data bus.
When specifying timing and other performance parameters, manufacturers will normally use a conservative approach. More specifically, the actual performance capabilities of integrated circuits will generally vary over a significant range for a variety of reasons, such as process variations. The performance parameters for the integrated circuits will generally be specified at values that are sufficiently conservative for substantially all of the integrated circuits to fall within the specified range of performance capabilities. However, most of the integrated circuits are actually capable of performing at a significantly higher level. As a result, many integrated circuits are sold as relatively low performance devices at a relatively low price even thought they actually perform at a significantly higher level and could be sold at a significantly higher price.
One approach to specifying the performance parameters of integrated circuits in a more advantageous manner is to "speed grade" the integrated circuits after manufacture. In speed grading, integrated circuits with a relatively broad range of performance values are tested and then grouped according to their performance during testing. Integrated circuits that are found to perform at a higher speed are specified with more stringent time parameters, while integrated circuits that are capable of operating only at slower speeds are specified at less stringent parameters. Using this speed grading approach, the same integrated circuits manufactured in the same process will have different performance specifications. However, all of the integrated circuits can be sold at a price commensurate with their actual performance.
Although speed grading allows integrated circuits to be used at their highest potential, there are nevertheless problems incurred using this approach. First, it may be difficult for a user or system designer to determine the performance parameters that are associated with any particular integrated circuit. Generally, the performance parameters for various speed grades are specified in a data book in the same manner that performance parameters are specified for non-speed graded integrated circuits. A marking on an external surface of the integrated circuit identifies its speed grade. The performance parameters for the integrated circuit are then determined by looking up the performance parameters in the data book that correspond to the speed grade marked on the integrated circuit. Unfortunately, users may not always have an up-to-date data book available. Also, the speed grade markings on the integrated circuit can become obliterated. Under these circumstances, it may not be possible to determine the performance parameters of the integrated circuit.
Another problem in speed grading integrated circuits results from the difficulty in specifying performance parameters under circumstances where the performance parameters vary depending upon the frequency of a clock signal used to synchronize the operation of the integrated circuit. To determine the performance parameters of an integrated circuit operating in synchronism with a clock signal of a predetermined frequency, different sets of performance parameters are specified for respective clock frequencies. Again, however, it is necessary for the user to find the appropriate set of performance parameters in the data book to determine the performance parameters of the integrated circuit operating at the chosen clock frequency.
There is therefore a need for an improved technique for determining the performance parameters of an integrated circuit, particularly where the integrated circuit is to be operated at optimum performance at a clock frequency that can be chosen by the user of the integrated circuit.